Book of jasher?

I can already see the rubber room goons coming with their butterfly nets lol.

Having noticed it sited a few times in a pretty famous book, I went and found it online and started reading it. It's not a bad read, and I would imagine a few gents here may have picked it up at some point. Aside from expanding the dialogue of some well known accounts, what's the deal with this book? Is it legit? I didn't know it was a real book until two days ago, I thought it was sort of legendary.